Stoves for burning fuel in the form of pellets manufactured from biomass are known to provide acceptable alternative heat sources for conventional heating units such as gas, electric and oil furnaces. Such stoves generally include a sealable firebox into which is fed fuel and air or other gases to support the combustion of the fuel. Stoves for residential heating utilize either a top feed mechanism that delivers the pelletized fuel onto a grate or a bottom feed system that forces the pellets into a burn pot from below. The top feed system is generally considered to be preferable due to its simpler design. In a top feed system, in order to provide sufficient amounts of combustion gases to the fuel, the grate onto which the fuel is deposited includes a perforated plate wherein the combustion gases pass through the perforations into the burning fuel. The major drawback of the top feed system has been the inability to remove the non-combustible ash and clinkers from the grate after combustion of the pellets. The accumulation of the ash and clinkers is troublesome because it eventually blocks the flow of air through the perforations in the grate and into the fire. This results in reduced heat output and burning efficiency.
Accordingly, there is a need for an improved grate and grate assembly which provide the advantages described above with regard to perforated grates, without suffering from the drawbacks associated with the accumulation of non-combustible ash and clinkers. A suitable grate and grate assembly would allow for the effective removal of non-combustible ash and clinkers from the grate to prevent clogging of the perforations in the grate.
Another drawback of pellet-burning stoves that employ top feed systems is the difficulty in maintaining the fuel in a compact volume for efficient combustion, particularly at low feed rates. With low feed rates, there is a tendency for the fuel pellets to spread out and form a thin layer. The combustion of fuel pellets in a thin layer is generally less efficient than combustion of fuel pellets that are maintained in a compact volume.
Accordingly, there is also a need for an improved grate and grate assembly that employs a perforated grate and is designed to concentrate and maintain the fuel pellets in a compact volume so that the efficiency of combustion at low feed rates is high.
Grates and grate assemblies that concentrate and maintain fuel in a compact volume for efficient combustion at low feed rates must also be suitable for combustion at high feed rates. At high feed rates, in certain grates designed to concentrate fuel, there is a tendency for the fuel to build up to volumes and depths that hinders the ability of the grate to effectively remove ash and clinkers. For example, in top feed systems, one of the factors that contributes to the removal of ash and clinkers through the perforations of the grate is the breakup of clinkers by the force of fresh fuel pellets falling on the pile of partially or fully combusted fuel. If the grate allows the fuel pellets to build up to an excessive depth, the force of the falling fuel pellets is not transmitted to the bottom of the fuel pile where the clinkers are most prevalent.
Accordingly, there is a need for an improved grate assembly that overcomes the foregoing problem of excessive fuel buildup with the consequence of reduction of clinker breakup.
Still another drawback of pellet-burning stoves is that it is often difficult to light the pellets and ensure that they remain burning. Pellet fuel must be raised to a very nigh temperature to initiate combustion, and occasionally requires relighting to maintain the combustion. Although the lighting of the pellets may be manually performed, the time and attention required to light the pellet fuel may occasionally be burdensome. Other ignition devices exist for automatically bringing pellet fuel to a sufficient temperature to cause the pellets to ignite. Such ignition devices typically rely on a flow of air through a sheath surrounding a metal rod that is heated to a very high temperature. The air is heated by the rod to a temperature that causes the pellets to ignite. Although automatic ignition devices simplify the lighting of the biomass pellets, they typically require additional fixtures in the grate in order to mount the ignition device above or below the grate surface. Additionally, automatic ignition devices require a flow of air, which is not always available or consistent in all grate environments.
Accordingly, there is a need for a grate assembly for a pellet stove that incorporates a means for lighting the pellets supported on the grate assembly.